Abstract: The invention is a sensor for the presence of bio-specific (e.g., immunological) molecules. It is aimed to giving an alternative, highly advanced method for performing different tests for the presence of immuno-specific molecules in liquid environments such as body liquids, biological cultures, environmental samples, etc. Gold patterns are photolithoghraphyically fabricated onto glass substrates to form addressable electrodes of micron size. The sensor is assembled when colloidal particles from suspension are deposited dielectrophoretically in the microscopic gaps between the electrodes. The surfaces of these particles carry immuno-active binding sites that collect the target molecules. The sensor readout is accomplished by secondary tagging of the target molecules with colloidal gold and its enhancement by silver nucleation, which leads to short-circuiting of the electrodes. The device allows extreme miniaturization and direct electric readout. We anticipate detection levels as low as 10.sup.-21 M, which is a 200 times gain in sensitivity over the conventional techniques.